Recently, as interest in energy storage technology is increasing and the technology has been widely applied to various fields such as mobile phones, camcorders, notebook PCs and electric vehicles, efforts for research on and development of electrochemical devices have materialized.
In this respect, electrochemical devices have attracted the most attention, and interest in secondary batteries that can be charged and discharged among these is increasing. Particularly, among secondary batteries that are currently being used, a lithium secondary battery developed in the early 1990s is getting the most attention due to its high operating voltage and superior energy density.
The lithium secondary battery is composed of a negative electrode made of a carbon material that can occlude and release lithium ions, a positive electrode made of a lithium-transition metal oxide, and a non-aqueous electrolyte.
The lithium secondary battery may be divided into a lithium ion liquid battery (LiLB) using a liquid electrolyte, a lithium ion polymer battery (LiPB) using a gel-type polymer electrolyte, a lithium polymer battery (LPB) using a solid polymer electrolyte, and the like according to the type of used electrolyte.
Recently, as the lithium secondary battery has been widely applied to various fields, there is an increasing demand for a lithium secondary battery that can be safely charged even at high voltage while maintaining excellent cycle lifespan characteristics even in harsher environments such as high or low-temperature, high-voltage charging, or the like.
Meanwhile, as the charging and discharging of a secondary battery proceed, the structure of a positive electrode active material is destroyed, and thus performance of a positive electrode is degraded. Also, when the structure of a positive electrode is destroyed, metal ions eluted from a surface of a positive electrode are electrodeposited on a negative electrode, and thus the negative electrode is deteriorated. Such a deterioration of battery performance tends to be further accelerated when the potential of a positive electrode increases or a battery is exposed to high temperature.
In order to solve the above problems, a method in which a material which protects a positive electrode by forming a film on the positive electrode is added to an electrolyte has been proposed.